We are studying virus morphogenesis, using bacteriophage gamma as a model system. Our work focusses on several virus assembly steps, dealing especially with DNA packaging. The first aspect of virus assembly we are studying is genome recognition. All viruses face the problem of encapsidating the virus chromosome from a nucleic acids. The general solution to the problem is that the virus produces a recognition protein that specifically binds to the viral genome. For gamma, recognition is accomplished through the action of terminase, the viral DNA packaging enzyme. Terminase specifically binds to a site (cosB) on gamma DNA and to the prohead, the empty protein shell into which DNA is packaged. We are interested in the interaction of terminase with both he DNA and the prohead. The second aspect is DNA processing. Many viruses produce progeny DNA is the form of linear polymers that are cut to unit length during packaging. For gamma,DNA processing is also one of the functions of terminase, which introduces staggered nicks, at cosN, to generate the cohesive ends found on virion DNA molecules. The third aspect is the molecular basis for ordered protein-protein interactions during assembly. Viral structures, such as the capsid, are produced in an ordered series of reactions. In a number of cases, such as for the phage T4 tail, the order of assembly is well worked out. The general explanation for how sequential ordering of reactions is controlled is that when a subunit adds to a growing structure, a reactive site is formed for binding the next component, which in turn generates a binding site for the next subunit. Little is known about these subunit-subunit interactions at the molecular level. Terminase binds the prohead, and apparently is activated to do so and we have begun such a study. Fourth, for viruses that have a lysogenic (or latent) cycle. We have encountered situations in which regulation of the expression of terminase is apparently disrupted, leading to the loss of potential lysogens. It appears that are tight controls on the expression of terminase quite apart from repression, and these are being examined. Firth, injection of gamma DNA into the host cell is not well understood. There exists evidence that specific DNA: protein interactions are important for injection, because injection is very poor for DNA molecules that have a substitution that replaces the normal right chromosomal end. We have isolated a point mutation affecting injection and can therefore begin detailed studies.